How to keep a bone in-between other bones?
You need 4 constraints:


Copy 100% Location from 1st bone
Copy 50% Location from 2nd bone
Copy 100% Rotation from 1st bone
Copy 50% Rotation from 2nd bone


But if the reason for the middle bone is to fix the deformation, you should fix that in the weight painting. If the reason is to keep the volume of the middle part, then it is ok.